The present invention relates to an electrical heating device, and in particular to a PTC heating device, for a vehicle, and to a method for electrically heating a vehicle.
Besides the known heating/cooling circuits with corresponding heat exchangers, increasing use has been made of electrical heating devices in recent years for heating the passenger compartment of a vehicle. These can be used both as units for assisting known heat exchangers, and as units for replacing them. In order to convert electrical energy into heat, conventional heating resistors and so-called positive temperature coefficient thermistors (hereafter PTC heating thermistors) are generally used. It is also known to combine both types of resistors, see e.g. FR-A-2 690 112.
PTC heating thermistors have a resistance that changes significantly as a function of temperature. Above a threshold temperature, it increases steeply so that PTC heating thermistors provide a certain degree of self-regulation in the operating range, for which reason the use of PTC heating thermistors has gained popularity in recent years.
FR-A-2 742 384 discloses a heating device of the generic type having the features of the preamble of claim 1. This known device comprises a heat-exchanger body whose configuration is chosen in order to integrate PTC thermistors. The PTC thermistors are arranged and connected in groups. Individual or multiple groups can be turned on selectively by means of a circuit, e.g. in order to provide different temperatures for different regions of the heat exchanger.
Since, owing to different vehicle operating conditions, the levels of electrical power vary, very substantial power losses occur in the known devices, and there is also a risk of overloading the vehicle electrical system. In order to solve the aforementioned problems, various attempts have been made to provide an improved control system for PTC heating elements. These include, inter alia, the relay-controlled switching of heating circuits with equal power, as e.g. indicated in FR-A-2 742 384 cited above. Although this does provide at some degree of improvement in terms of the power loss that occurs, it cannot be regarded as any more than an interim solution.
An alternative approach consists in controlling the power continuously on the basis of pulse-width modulation. Although this method promises a low corresponding power loss, it unfortunately entails the disadvantage of additional sources of interference. The use of the linear controller has also been found to be unfeasible owing to the very high power losses.
The object of the invention is accordingly to develop an electrical heating device of the generic type, and a method of the generic type for electrically heating a vehicle, in such a way as to minimize the power loss, with the further intention of guaranteeing as continuous as possible an adjustment of the heating power. The device and the method are also intended to contain the fewest possible sources of interference, or to function with the least possible susceptibility to interference.
According to one aspect of the invention, there is provided an electrical heating device, in particular a PTC heating device, for a vehicle, having a plurality of heating resistors, in particular PTC heating thermistors, which are connected in groups, and having a circuit by means of which one or more groups can be turned on selectively, wherein the circuit turns on a selection of the groups at least on the basis of the available electrical power.
According to another aspect of the invention there is provided a method for electrically heating a vehicle by using electrical heating elements, in particular PTC heating elements, which are connected in groups, with the step of turning on one or more groups, wherein the said step of turning-on is carried out in relation to a selection of groups, at least on the basis of the available power.
The electrical heating device according to the invention is, in particular, preferably a PTC heating device for a vehicle. It comprises a plurality of heating resistors, in particular PTC heating thermistors, which are connected in groups. It further comprises a circuit by means of which one or more groups can be turned on selectively. The device according to the invention is distinguished in that at least two of the groups have different rated heating powers, and in that the circuit turns on, or supplies with electricity, a selection of the groups at least on the basis of the available electrical power.
The method according to the invention for electrically heating a vehicle by using electrical heating elements, in particular PTC heating elements, which are connected in groups, with the stept of turning on one or more groups, is distinguished in that at least two of the groups have different heating powers, and in that the stept of turning-on is carried out in relation to a selection of groups, at least on the basis of the available power.
The device according to the invention, and the method according to the invention, hence make it possible to select groups with different heating power, on the basis of the available electrical power, in order to obtain optimally efficient heating of the passenger compartment at all times, in particular in the operating range of the individual groups of PTC heating thermistors. Accordingly, overloading of the electrical system built into the vehicle and the occurrence of power losses can be prevented to all practical intents and purposes.
In order to make it possible to obtain a heating function even when the available electrical power is relatively low, the group with the lowest heating powers should advantageously provide a temperature increase, for the medium to be heated up, in particular air, of at most 1 kelvin in the operating range of the group of PTC heating thermistors.
Advantageously, the maximum difference in rated heating power between at least two groups provides a temperature increase, for the medium to be heated up, of at most 1 kelvin in the operating range, so that the heating power can be controlled continuously to all practical intents and purposes.
In a preferred embodiment, a total rated heating power graduation is provided, in particular with an increment which corresponds to a respective temperature increase, for the medium to be heated up, of at most 1 kelvin in the operating range, by means of respective selection. In other words, groups of PTC thermistors, or even conventional heating resistors, can be combined in such a way that various total rated heating powers can be obtained.
In a particularly preferred embodiment, the heating power of at least one group substantially satisfies the equation
Pn=P1+(nxe2x88x921)xc3x97P1 
where, in a series of groups with different rated heating power in order of increasing heating power, n denotes one of the groups. Consequently, for example, the desired graduation of the total heating power may be obtained, in the case of four groups, by providing the first group with a rated power of 100 watts, a second group with a rated power of 200 watts, a third group with 300 watts and a fourth group with 400 watts of electrical power. In the example indicated, it is therefore possible to obtain total heating powers of up to 1000 watts with a 100-watt interval. If e.g. an electrical power of 900 watts is available from the vehicle electrical system, and the user desires and sets a maximum heating power, the groups with respective powers of 200 watts, 300 watts and 400 watts may be operated together, so as to provide an overall optimized efficiency.
In an alternatively preferred embodiment, the rated heating of a group satisfies the following equation
Pn=2(nxe2x88x921)xc3x97P1 
where again, in a series of groups with different rated heating power in order of increasing heating power, n denotes one of the groups. Assuming a rated heating power P1=100 watts for the first group, corresponding approximately to a temperature increase of 1 kelvin in the air flowing past, the group with the next heating power up has a rated electrical power of 200 watts. The following groups should then approximately have electrical powers of 400 and 800 watts. With this configuration of the individual groups as well, it is possible to graduate the total heating power with an increment equal to the smallest rated heating power, the maximum total rated heating power, if the vehicle can provide sufficient power, amounting to 15 times the rated heating power of the group with the lowest power.
Both in the device according to the invention and in the method according to the invention, the turning-on of individual groups may be carried out, as well as on the basis of the available electrical powers, also on the basis of sensor signals and/or of user-setting signals. The signals provided by means of sensors may, inter alia, comprise external temperature signals, passenger-compartment temperature signals and the like. The user-setting signals may, inter alia, cause the entire device to be switched on and off, or may lead to the desired thermostatic regulation within the limits of the available electrical power.